Self acting aerodynamic bearings have been known since 1940 which comprise a plain cylindrical horizontal shaft which is rotatable in a plain cylindrical bearing with a small radial clearance between the two. The pressure profile within the bearing is modified by the nature of the shearing of a gas film as it passes through the convergent/divergent passage, the passage being created automatically by the weight of the shaft.
As the speed of the shaft is increased the shaft moves progressively towards, but never quite reaching, a truly concentric condition. At a certain speed, the bearing has the disadvantage that a serious instability suddenly develops where the centre of the rotating shaft commences to rotate about the centre of the bearing at half the rotational speed of the shaft. At this point, the mechanism which created the distortion of the uniformity of pressure around the shaft is destroyed with the result that load capacity is totally lost and very rapid seizure ensues.